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UNDERWATER COMMUNICATION
THROUGHT MAGNETIC INDUCTION (MI)
by
Sana Ramadan
Submitted in partial fulfilment of the requirements
LIST OF TABLES ...................................................................................................................................................... iii
LIST OF FIGURES .................................................................................................................................................... iv
ABSTRACT ................................................................................................................................................................... v
LIST OF SYMBOLS USED ...................................................................................................................................... vi
ACKNOWLEDGEMENTS ..................................................................................................................................... viii
CHAPTER2 MI COMUNICATION .................................................................................................................... 3
2.1 Related Work ..................................................................................................................................................... 3
CHAPTER4 RESULTS AND DISCUSSION .......................................................................... 18
4.1 MI Channel ........................................................................................................................................................ 18
Table 12 Flat attenuation uplink and downlink (SNR)-Fresh water (Example2) .......... 48
iv
LIST OF FIGURES
Figure 1 Inductive transmitter and receiver [4] .................................................................................. 12 Figure 2 Magnetic moment [5] ............................................................................................................. 13 Figure 3 Model of a magneto-inductive communication system .................................................... 16 Figure 4 Illustration of a loop antenna at the transmitter side .......................................................... 18 Figure 5 Illustration of the second loop at the receiver ..................................................................... 21 Figure 6 Skin depths as a function of frequency and conductivity for both seawater and fresh
water ................................................................................................................................................. 26 Figure 7 Induced received voltage as a function of frequency for different separation distances
between Tx and Rx ......................................................................................................................... 27 Figure 8 Induced voltages for a coaxial receiver loop as a function of frequency for different
separation distances between Tx and Rx ....................................................................................... 28 Figure 9 Distance and frequency dependent attenuation of the magneto-inductive signal for a
typical value of σ=4S/m for seawater .......................................................................................... 29 Figure 10 Illustration the channel attenuation in seawater with σ = 4s/m and in fresh water
with σ = 0.01s/m .......................................................................................................................... 31 Figure 11 System transfer function Hf with different penetration depths for both seawater and
fresh water ........................................................................................................................................ 32 Figure 12 SNR for various communication ranges and different penetration depths. ................. 35 Figure 13 SNR for seawater, figure shows two conditions when Fkϕ < 0.5, and Fkϕ > 2 ...... 36 Figure 14 Example 1 on uplink and downlink communications ..................................................... 39 Figure 15 Flat attenuation uplink (SNR)-Seawater (Example1)...................................................... 41 Figure 16 Flat attenuation uplink (SNR)-Fresh water (Example1) ................................................. 42 Figure 17 Flat attenuation downlink (SNR)-Seawater (Example1) ................................................ 44 Figure 18 Flat attenuation downlink (SNR)-fresh water (Example1) ............................................. 45 Figure 19 Example 2 for uplink and downlink communication ...................................................... 46 Figure 20 SNR comparison for up-link and downlink operation .................................................... 49
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ABSTRACT
Wireless Underwater Communication Networks (WUCNs) have recently become a hot
topic of research due to applications such as mine detection, navigation, and pollution
monitoring. One-third of the earth’s surface is covered by water, making it a challenging
environment for communications. Acoustic waves are currently the most common
technology used in underwater communications, but this approach suffers from large
attenuation and propagation delays. Less commonly used are electromagnetic waves (EM),
which experience range limitations in water, and optical waves, which encounter
scattering. In this thesis, we will focus on magnetic induction-based communication.
Magnetic induction (MI) has several advantages compared to the commonly used acoustic,
EM, and optical communication methods. For instance, MI does not suffer from multi-path
fading or scattering, and the signal propagation delay is negligible. We will roughly
estimate the achievable range, operation frequencies, bandwidth, path loss, capacity and
distortions of the MI in conductive media such as water.
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LIST OF SYMBOLS USED
𝛿 Skin depth
휀 Electrical permittivity or dielectric constant
휀0 Electrical permittivity of free space ≈ 8.85 × 10−12 𝐹𝑚−1
휀𝑟 Relative permittivity
𝜆 Wavelength
𝜇 Magnetic permeability
𝜇0 Magnetic permeability of free space ≈ 4𝜋 × 10−7 𝐻𝑚−1
𝜇𝑟 Relative permeability
𝜌 Mass density
𝜎 Electrical conductivity
𝜙 Specific aperture (merit factor)
𝜔 Angular frequency, 𝜔 = 2𝜋𝑓
𝐵 Flux density
𝑐 Speed of light, ≈ 3 × 108 𝑚/𝑠
f Frequency
𝐹𝑎 Atmospheric noise temperature
𝐹𝑘 Atmospheric noise
𝐻 Magnetic field
𝑘 Boltzmann constant, ≈ 1.38 × 10−23𝐽𝑘−1
𝑘0 Wave number for free space
𝑚𝑑 Magnetic moment
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𝑁 Noise voltage or power
𝑆 Signal voltage or power
𝑇 Temperature [K]
𝑇𝑎 Atmospheric noise temperature
𝑍0 Wave impedance of free space, 𝑍0 = √𝜇0
0
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ACKNOWLEDGEMENTS
I would like to express my appreciation to my supervisor Schlegel who has
cheerfully answered my queries, provided me with materials, checked my
examples, assisted me in a myriad way with the writing and helpfully
commented on earlier drafts of this project. Also, I am very grateful to my
family and friends for their support throughout the production of this project.
CHAPTER1 INTRODUCTION
Traditional wireless networks that use EM suffer from high path loss, which limits their
communication range. To increase the EM range, a large antenna is used for low
frequencies, but this is unsuitable for small underwater vehicles. Optical waves experience
multiple scattering, limiting the application of optical signals to short-range distances.
Additionally, the transmission of optical signals requires a direct line of sight, which is
another challenge for mobile underwater vehicles and robots [10]. Radio waves suffer from
large attenuation and their range is limited to skin depth, which is associated with water
conductivity. Seawater has high conductivity, but the salinity and physical properties of
each type of seawater differ. Seawater measures around 4 S/m, whereas for pure water,
typical values range between 0.005 and 0.01 S/m [4].
Acoustic waves are widely used in underwater communication because they can travel long
distances. However, acoustic signals suffer from low bandwidth and data rates as well as
large propagation delays because the sound speed equals 1500 m/s [10]. Magnetic
induction (MI), however, is a promising solution for underwater short-range
communication.
MI technology has already proved to be a useful tool for underwater exploration. Due to
the high velocity of MI propagation, frequency offsets due to the Doppler effect are
negligible. Time-varying magnetic induction, generated by a primary coil or loop and
sensed by a secondary coil or loop, is the fundamental basis for magneto-inductive
communications. Transformers consisting of two coils positioned very close to each other
are the most common application of magnetic induction.
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Ideally, total power is transferable from one coil to another. However, a regular magneto-
inductive communication system featuring a large distance between primary and secondary
coils rapidly experiences a power drop to a very small fraction. For this reason, coils are
usually asymmetric, meaning that the transmitter coils are large and heavy, whereas the
receiving coils are tiny and light [2]. A steering current drives the primary coil which
generates a reactive magnetic field and makes MI communication possible. By using such
a method, the energy stays local and is not transferrable at large distances that typical radio
waves can propagate. Thus, because the receiving signal strength will experience a rapid
decline over communication distance, it pushes MI communications out of the running as
an alternative to regular wireless communications.
MI communication has the potential to be used for underwater communications, as less
power loss occurs in comparison to electro-magnetic (EM) radio waves, which are rapidly
absorbed in water. From a theoretical communication perspective, MI channels do not
differ significantly from other electro-magnetic communication channels, and our
approach is generally standard in communication theory. However, there are some
differences between MI and RF communications, one of which is focusing on different
physical characteristics of the environment when developing practical channel models for
MI communications.
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CHAPTER2 MI COMMUNICATION
MI communications use low-frequency modulation to enable reliable communication in
areas where traditional radio-frequency (RF) communications fails. These environments
include areas with a high concentration of conductive elements, through highly reflective
barriers such as the surface of water and communicating through the earth. So far, MI
communications has been limited to short ranges and low data rates but shows promise for
providing high-data throughput at short ranges. The communication scenario provides
unique opportunities and challenges. Additionally, given the nature of MI, it has not been
fully explored, which means that current systems operate far below capacity.
2.1 Related Work
Magnetic induction was first introduced as an underwater tool in [6], which featured a high-
speed link over a short range. In [8] and [1], underwater magneto-inductive networks were
analyzed using mutual coupling. Developing a model for magneto-inductive
communication was addressed in [7] by demonstrating the coupling between the coils in
the near field. In [3], a narrow bandwidth of a few KHz was reported because of the high
Q-factor (quality factor) of the magnetic coils. To expand the bandwidth [3], the front-end
resonant frequency was modulated in [14]. In [10], tuned resonant circuits (narrowband)
or unturned circuits (wideband) were applied to achieve high efficiency. Most of the
literature, such as [13], [9] and [12], demonstrates channel modeling from an end-to-end
perspective. So, for example, they focus not only the channel medium but also
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on the characteristics of the transceiver and the coils. In this study, we focus on the channel
as a medium.
2.2 Background
“Near field magnetic induction system is a short-range wireless physical layer that
communicates by coupling a tight, low-power, non-propagating magnetic field between
devices. The concept is for a transmitter coil in one device to modulate a magnetic field
which is measured by means of a receiver coil in another device” [4]
In magneto inductive communications system, the distance of coils is usually larger, and
transferred power drops off sharply to a very small fraction, so the coils should be
asymmetric, which the transmitter coil being heavy and large, and the receiver coils being
light and small. We are working in conductive media such as water, where the electrical
conductivity 𝜎 leads to energy dissipation of the material because of the eddy current
which generates strong secondary field. Electrical conductivity gives a measure of a
material’s ability to conduct an electric current.
Figure 1 Inductive transmitter and receiver [4].
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The principle of magnetic induction is the current in the primary coil (transmitter) generates
magnetic field then the magnetic field induces current in to secondary coil (receiver).
Magnetic flux is a general term associated with a field that is bound by a certain area. So,
magnetic flux is any area that has a magnetic field passing through it. Electron has a
magnetic dipole moment. It's close to an electric dipole moment because it generates a
magnetic field that behaves similarly to an electric dipole field (falls off like 1 /r3). The
lowest order moment possible in magnetism that obeys Maxwell's equations is the dipole
moment.
Figure 2 Magnetic moment [5].
According to Figure 2 When the current (I) traveling around the edge of a loop of cross
sectional area (A=𝜋𝑟2,where 𝑟 is the loop radius), the magnetic moment will produce. The
physics formula for calculating the dipole moment of a flat current carrying loop of wire
is:
𝑚𝑑 = 𝑁𝐼𝐴 (1)
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where 𝐼 is the current (Amperes) in the 𝑁 turns, 𝐴 is the area (m2) of the loop, and 𝑁 is the
number of windings in the coil. The magnetic dipole moment (𝑚𝑑 ) is a vector whose
direction is perpendicular to A and determined by the right-hand rule. Thus, the unit of
magnetic moment is (A m2).
From (2), magnetic moment can be increased by increasing either𝑁, 𝐼, 𝐴. However, by
increasing 𝑁 leads to more coupled core loss, as well as by increasing𝐼, resulting more
power losses (𝐼2𝑅), also, increasing cross section area 𝐴, can lead to increase diameter and
large antenna size is not practical for most applications. To increase magnetic field
strength 𝐻, magnetic moment 𝑚𝑑 could be enhanced by using magnetic permeability of
the coil, given as,
𝑚𝑑 = 𝜇𝑒𝑁𝐼𝐴 (2)
Magnetic permeability 𝜇 determines the extent of magnetization obtained by the material
in the presence of an external magnetic field and denoted by 𝜇 = 𝜇0𝜇𝑒, where 𝜇0 is the
permeability of free space, while 𝜇𝑒 is relative permeability of material, which various
depending upon material type.
Clearly that magnetic moment depends on the number of turns, however the magnetic
moment in relation to the mass and diameter, it is independent of the number of turns. For
example, if we considering two magnetic loops A and B, loop A has 100 turns of wire,
carrying a current of 4 A, with resistor 2Ω. we will compare loop A with loop B, where
loop B has 500 turns and assuming both of loops have an identical mass.so loop B will
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have five times the length of wire and resistance will be 25 times greater that is because
five times greater due to length and another five times greater due to small cross section
area. So, to get the same magnetic moment as loop A, it needs only 1/5 of the current (i.e.
0.8 A). Resulting in power dissipation 𝐼2𝑅 for loop A is 32w as well as for loop B is 32w.
The magnetic field strength =𝑚𝑑
4𝜋𝑟3 , at a coaxial point at distance 𝑟 from the loop antenna
is proportional to the magnetic moment and decreases with the third power of 𝑟. Magnetic
field strength can be increased by increasing magnetic moment.
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CHAPTER3 METHODOLOGIES
The methodologies we are adapting here quite standard in the field of communication
theory.
3.1 The Magneto Inductive Channel
The MI channel fundamentally includes the MI transmitter, the magnetic channel in
conductive media, and the MI receiver. These are shown in the block diagram in Figure 3
and discussed individually below.
Figure 3 Model of a magneto-inductive communication system.
3.1.1 The Magneto Inductive Transmitter
An inductive loop is the most practical and efficient way to generate a magnetic field [2].
The magnetic field from the loop antenna falls into the following three regions: the near
field region, in which the field is described by quasi-static equations and there is no
significant radiation as the electric and magnetic fields are in phase quadrature; the far
field, which represents the region where the electric and magnetic fields are in phase and
where
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the magnetic field peels off as a propagating electro-magnetic wave; and the transition
region, which is located between the near and far field regions [2].
Table 1 Showing the boundary conditions of the near field, transition zone, and far-
field.
As we can see from Table 1 boundary conditions of the three regions using two common
measures (wavenumber |𝒌𝟎|and skin depth 𝜹), where is the 𝑻 the ratio of distance to skin
depth, given as 𝑻 =𝐫
𝛅 and 𝒌𝟎 =
𝟐𝛑𝐟
𝐜. We are primarily interested in the near field, for which
𝐫 < 𝛅 and
T =r
δ< 1 (3.1)
where r is the operating distance and T is skin depth numbers.
The magnetic near-field decays with the inverse cube of the distance. According to the Biot
– Savart Law, the magnetic field from a small element is proportional to (1
𝑟3).
𝑩 =𝝁𝟎
𝟒𝝅∫ 𝑰
𝒅𝑰×𝒓
𝒓𝟑 (3.2)
where 𝐵 is the flux density and 𝐼 is the current (see Figure 4).
Properties
Near–field
approximation
Transition region
approximation
Far-field
approximation
Skin depth 𝑇 ≪ 1 𝑇2 ≪ 1 𝑇 ≫ 1
Wave number 𝑘0𝑟 ≪ 1 (𝑘0𝑟)2 ≪ 1 𝑘0𝑟 ≫ 1
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Figure 4 Illustration of a loop antenna at the transmitter side.
The magnetic field strength for induction at a distance r from the loop antenna is
proportional to the magnetic moment and decreases with the third power of 𝑟, given by
H =md
4πr3 (3.3)
where 𝑚𝑑 is the magnetic moment for a circular loop antenna given by
𝒎𝒅 =𝒅𝒕
𝟒√𝑴
𝝈
𝝆√𝒑 = 𝝓𝒕√𝒑 (3.4)
𝑝 is the dissipated power, and 𝜙𝑡 is the antenna merit factor, given by
𝝓𝒕 =𝒅𝒕
𝟒√𝑴
𝝈
𝝆 . (3.5)
In the above, 𝜙𝑡 depends on the mass 𝑀of the coil, the diameter𝑑𝑡, the electrical properties
of the wire material, which are the conductivity 𝜎 , and the material density 𝜌 . These
parameters are collected in the antenna merit factor 𝜙𝑡, which has dimensions m2/√Ω. The
magnetic moment also depends on the dissipated power 𝑝, but does not depend on the
number of turns [3].
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For example, consider a portable induction loop antenna, where the diameter is 1m and
mass is 0.6 Kg. If we take into account that the loop is made from copper √𝜎
𝜌 =8m/Kg√Ω,
the magnetic moment is 50 Am2 with a power dissipation of 10 w.
The 𝐻-field is directly proportional to the loop current. Hence, since the power loss in the
antenna (which is due to ohmic resistance) is also directly proportional to the loop current,
there is no fundamental preference of frequency [2].
3.1.2 The Communications Channel
Communication through conductive media such as seawater is possible only over distances
of a few skin depths. The skin depth gives a measure of the penetration of magnetic field
into a given medium. The magnetic field decays with distance into the medium, and this
decay of the field is expressed by the skin depth, given by [3]